This invention relates to a taper cutting method and, more particularly, to a taper cutting method for inserting a cut surface having a frustoconical shape between first and second taper-cut surfaces.
As is well-known in the art, a wire-cut electric discharge machine has a wire stretched between an upper guied and a lower guide and machines a workpiece by producing an electrical discharge between the wire and the workpiece. The workpiece, secured to a table, is transported in X and Y directions along a machining contour in response to commands from a numerical control apparatus. When the wire is tensioned normal to the table (workpiece), the upper and lower surfaces of the workpiece will be machined into contours which are identical. If the arrangement is such that the upper guide can be displaced in the X and Y directions (referred to as the U and V axes) to incline the wire with respect to the workpiece as by displacing the upper guide in a direction at right angles to the direction of workpiece movement, then the upper and lower surfaces of the workpiece will not be machined to the same contour, and the surface cut by the wire will be inclined. This is so-called taper cutting.
FIG. 1 is a view for explaining such taper cutting, in which a wire is stretched between an upper guide UG and a lower guide DG at a predetermined angle of inclination with respect to a workpiece WK. If we take the lower surface PL of the workpiece WK as the programmed contour (the upper surface QU of the workpiece WK may also serve as a programmmed contour), and if we let .alpha. denote the taper angle, H the distance between the upper guide UG and the lower guide DG, and h the distance from the lower guide DG to the lower surface of the workpiece WK, then the offset d.sub.1 of the lower guide DG and the offset d.sub.2 of the upper guide UG with respect to the lower surface PL of the workpiece, may be expressed as follows: ##EQU1## Note that d is the cut width.
Accordingly, if the movement of the upper guide UG between which the wire WR is stretched is so controlled in relation to workpiece movement that the offsets d.sub.1, d.sub.2 remain constant, then taper cutting at the taper angle can be carried out, as shown in FIG. 2. The dashed line and one-dot chain line in the Figure indicate the paths of the upper and lower guides UG, DG, respectively.
In performing taper cutting with such a wire-cut electric discharge machine, a programmed path on the upper or lower surface of the workpiece, feedrate on the programmed path, taper angle .alpha. and distances H, h, etc., are commanded as set forth above and cutting is carried out in the manner commanded.
Thus, in a wire-cut electric discharge machine, taper cutting can be performed through comparatively simple control if the taper angle .alpha. is constant.
With the above-described conventional method, however, corner cutting for providing a corner portion with any desired degree of roundness cannot be carried out in simple fashion and a corner having any radius cannot be cut at the upper and lower surfaces of the workpiece.